Download Uncertainty in Measurements

History of the Atom
Leading up to the current model
History of the Atom
2.1 The atom
1 hour
TOK: What is the significance of the model of the atom in the different areas of knowledge? Are the models and theories that scientists create accurate descriptions of the
natural world, or are they primarily useful interpretations for prediction, explanation and control of the natural world?
Assessment statement
Obj
Teacher’s notes
2.1.1
State the position of protons, neutrons and electrons in the
atom.
1
TOK: None of these particles can be (or will be) directly
observed. Which ways of knowing do we use to interpret
indirect evidence gained through the use of technology? Do
we believe or know of their existence?
2.1.2
State the relative masses and relative charges of protons,
neutrons and electrons.
1
The accepted values are:
2.1.3
Define the terms mass number (A), atomic number (Z) and
isotopes of an element.
1
2.1.4
Deduce the symbol for an isotope given its mass number
and atomic number.
3
2.1.5
Calculate the number of protons, neutrons and electrons in
atoms and ions from the mass number, atomic number and
charge.
2
2.1.6
Compare the properties of the isotopes of an element.
3
2.1.7
Discuss the uses of radioisotopes
3
Leading up to the current model
The following notation should be used:
, for example,
Examples should include 14C in radiocarbon dating, 60Co in
radiotherapy, and 131I and 125I as medical tracers.
Aim 8: Students should be aware of the dangers to living
things of radioisotopes but also justify their usefulness with
the examples above.
Models of the Atom
Earliest Models:

Dalton (and contemporaries)
“Billiard Ball”
Solid sphere
Cannot be divided up into smaller particles or pieces.
Atom neutral and no charge
Atoms of same element are made of the same types of
atoms.


Faraday
Suggested that structure of atom is somehow
related to electricity.

Long series of experiments:
Atoms contain particles that have electrical charge.
*
Models of the Atom (cont.)

Faraday (cont.)
Greeks knew that if you rubbed amber with a
cloth, it attracted dust or other particles.


Static electricity
Franklin
Studied Static Electricity
Famous Kite Flying experiment

Findings:
Object could have one of two electrical charges
Called them positive and negative (+, -)
Alike charges repel (+ +) and (- -)
Lightning was static electricity on a larger scale.
Models of the Atom (cont.)

Others (mid-1800s)
Scientists investigated electric currents
Cathode Ray Tube (CRT)
J.J. Thompson (1896)
Systematic studies on cathode rays Movie
concluded that:
Through experiments, cathode rays were
composed of negative particles and these negative
particles could be manipulated with magnet & electric
currents.
Atoms were not indivisible, solid sphere. But had
substructure(s).

Eventually, addition of “pinwheel” to the tube
showed that particles in beam had mass.
Uses of this Technology
*
Models of the Atom (cont.)
J.J. Thompson (cont.)
called negative particles “Electrons”
Was able to determine ratio of electron’s electrical
charge to mass (1.76 x 108 coulombs per gram)
Millikan (1909)
Oil Drop Experiment (Movie)
Measured the charge of an electron
 charged droplets with x-rays (negative charge)
Varied rate of falling by changing charge in two charged plates.
Calculated that the charge on each oil drop was a multiple of
1.60 x 10-19 C (coulombs)

Figured the charge for an electron must be 1.60 x 10-19

From this and Thompson’s ratio, calculated the mass of e-.
*
Models of the Atom (cont.)
*
What did this do to the idea of the Model?
This new research/data showed that the atom
was NOT a solid sphere. But had “parts” that had
a negative charge.
However, the overall atom was neutral in charge.
Therefore, there must be positive “parts” to
balance the negative “parts”.
Gave rise to the “Plum Pudding” Model.

Electrons (-) spread randomly throughout the atom. And
surrounded by randomly spread out positive (+) parts.
Models of the Atom (cont.)
Radioactivity

Becquerel (1896)
Accidentally discovered uranium sample was
radioactive (placed on photography film).
Radioactivity:


Spontaneous emission from atom
Curie (Marie and hubby Pierre)
Becquerel’s colleagues
Isolated 2 other radioactive materials

Radium and Polonium
Models of the Atom (cont.)
Radioactivity

Scientists soon after discovered several things:
Radioactivity accompanies fundamental changes in
an atom.


A chemical change happens as radiation is given off!!!
Rutherford (early 1900s)
Studied Radioactivity
Used 2 electrically charged plates (Movie)
Found that some radiation deflected towards negative
plate. Called it an ALPHA RADIATION.
Other radiation deflected towards positive plate. Called
it BETA RADIATION.
Later scientists found another radiation which was
undeflected by electrical charge. GAMMA RADIATION.
Both Alpha and Beta radiations were shown to be particles.

Models of the Atom (cont.)

*
Rutherford (cont.)
Concluded that atoms contain electrons, but were
electronically neutral
Alpha-scattering experiment (Gold Foil Experiment)
Movie
Models of the Atom (cont.)
Gold Foil Experiment (results)
A few particles were deflected off their path.
Some particles “bounced back”



Think of playing pool. Glancing hits vs. more direct hits.
Findings.
Most of the mass was concentrated in the core.
The positive charge was concentrated in the core.
Most of the area around an atom was “empty”.
Nucleus is small compared to the atom, but very
large compared to electrons.
Rutherford called this core, the nucleus.
Lead to new model:

Nuclear Model
*

Moseley
Student of Rutherford’s
Found that atoms of each element contained unique
positive charge in their nucleus.
Helped solve the mystery of what makes atoms of one
element different from another element.

Atomic Number = # of PROTONS
Proton is the positive charge within an atom’s nucleus.
Atoms are electronically neutral.

# of Protons (atomic #) = # of Electrons
*

Bohr (1913):
Niels Bohr came to work in Rutherford’s laboratory
Bohr asked to work on model because there were some
problems with the nuclear model
According to Bohr's model only certain orbits were
allowed
Visualized the atom like the solar system. (Electrons in distinct
“orbits” around the atom’s nucleus).

*
Bohr's model of the atom is important



introduced the concept of the quantum (levels)
Started to explain atomic properties.
However, Bohr's model needed revision
failed to explain the nature of atoms more complicated
than hydrogen.

It took roughly another decade before a new
more complete atomic theory was developed
the modern atomic theory.
Louis de Broglie introduces the wave/particle duality of matter (1921)

Traditional (classical) physics had assumed that particles were particles
and waves were waves
However, de Broglie suggested that particles could sometimes
behave as waves and waves could sometimes behave as particles the wave/particle duality of nature.

He suggested a simple equation that would relate the two:
Particles have momentum (p), waves have wavelengths (l) and the two are
related by the equation
l=h/p
h=Planck's constant = 6.634x10-34 Js
p=(mass)x(velocity)
This wave/particle duality of nature turned out to be a key to the new
atomic theory.
Werner Heisenberg elucidated the Uncertainty Principle (1923)
Classical physics had always assumed that precise location and
velocity of objects was always possible.
Heisenberg, however discovered that this was not necessarily the
case at the atomic level.
In particular, he stated that the act of observation interfered with the
location and velocity of small particles such as electrons.


This is the case because observation requires light and light has
momentum.
When light bounces off an electron, momentum exchange can occur
between light and the electron which means the electrons location and
velocity have been altered by the act of measurement.
This scenario has important implications to what we can
measure at the atomic level.
Erwin Schrodinger took the ideas developed by de Broglie,
Heisenberg and others and put them together in a single equation
that is named after him.
Schrodinger Equation
Hamiltonian represents the total energy of the system.
Solving this equation can, in principle, predict the properties and
reactivities of all atoms and molecules.
Unfortunately, it is extremely difficult to solve for any but the most
simple atoms and molecules. some essential conclusions:


I) Energies are quantized: Atoms and molecules cannot have any energy but
only certain energies. This means that energies are "quantized".
II) The orbitals, associated with each energy, determine where the electrons
are located.
*
•These orbitals can be seen as the "rooms" in which the electrons
in an atom "live".
•The quantum energies together with the orbitals can be used to
explain chemical properties and reactivities.

Electron Cloud Model
Electrons are located within
certain 3-D regions around
the nucleus called clouds.
Atoms

The practical “stuff”

Atoms Made up of:
Electrons
Negative charge
Around the nucleus
VERY small mass
Protons
Positive charge
In the nucleus
Accounts for large part of mass of an atom
Neutrons
No charge
In the nucleus
Accounts for large part of mass of an atom.
“Glue” of an atom

A force called the strong force opposes and overcomes
the force of repulsion between the protons and holds the
nucleus together. (Binding Energy.)
*

*
Atoms (cont.)
Ions





Atoms can gain or lose electrons
Become Ions (more or less electrons than the # of
protons)
Can be either POSITIVE or NEGATIVE
Charge of ion = # of Protons - # of Electrons
e.g. Magnesium (Mg) atomic # 12. Loses 2 e# of Protons
12
-# of electrons -10
+2
Mg2+
Try a few:
a) Ca loses 2 e-
Ca2+
b) F gains 1 e-
F1-
c)
As gains 3 e-
As3-

Atoms (cont.)
Isotopes

All about Neutrons
“Glue”
Mass is 1 a.m.u. (Atomic Mass Unit)
(Mass of Proton are each 1 a.m.u.)
Think about the charges in the nucleus
(Repelling + charges)
More “glue” is needed as the # of protons climbs



Same # of protons (why?) and different # of neutrons
e.g. Hydrogen
Hydrogen (1H) has 1 proton, 0 neutrons. Mass is 1 a.m.u.
Deuterium (2H) has 1 proton, 1 neutron. Mass is 2 a.m.u.
Tritium (3H) has 1 proton, 2 neutron. Mass is 3 a.m.u.
Mass Number:
Sum of isotope’s protons and neutrons.
*
*
Isotopes (cont.)
37Cl
Mass # 
Atomic #  17
What is that number (decimal) at the bottom under
the symbol?
WHY?
Average of the isotope’s mass
e.g.
12C
98.90% (of mass # 12)
13C
1.10 % (of mass # 13)
0.9890 x 12 = 11.868
0.0110 x 13 = 0.143
Average atomic mass: 12.011 amu
Radioactivity
Changes in the Nucleus
Radioactivity
Nuclear stability (instability)
Recall:
Protons (? Charge)
Therefore: REPEL each other
Neutron (? Charge)
“Glue” to hold Protons together
Strong Nuclear Force (the glue)


Nuclear stability (cont.)
From 1-20, approximate 1:1 Protons: Neutrons.
Beyond 20, more Neutrons.
83 and beyond, spontaneous emissions
Can’t hold together indefinitely
Falls apart
Called “Decay”
As a radioisotope tries to stabilize, it may transform into
a new element in a process called transmutation.
Not only too little “glue”, but also too much.
As a general rule, lighter & heavier isotopes (vs.
common isotope) are likely radioactive.

*

Nuclear stability (cont.)
The basic unit of measure for radioactivity is the curie,
named after Marie Curie.
A quantity of 1 curie (or 1 C) is 37 billion atoms decay
(disintegrate) in one second.

1C = 3.7 X 1010 disintegrations/sec.
If the rate of decay is greater than 37 billion atoms in one second,
then the source would have an activity greater than one curie
 if that source had fewer than 37 billion atoms decaying in one
second, its activity would be less than one curie.


Types of Radioactive Decay (basics)
Alpha
Beta
Gamma
Specials
Positron Emission
Electron Capture
Alpha
Consists of 2 protons & 2 neutrons
What is that?
4
2He
or
4
2a
Helium Nucleus (no electrons)
Penetration power:
Stopped by paper
Charge/Mass:
2+ / 4 amu
With Alpha, think LOSS
*

Types of Radioactive Decay (cont.)
*
Beta
Consists of high speed electrons
What is that?
e- or 0-1b
Where do they come from????
Neutron changing into a proton (flip of a quark), ejects ePenetration power:
Stopped by heavy clothing
Charge/Mass:
1- / ~0 amu
With Beta, think CHANGE

Types of Radioactive Decay (cont.)
Gamma

Consists of high energy photons
What is that?
g
Similar to X-rays
Penetration power:
Stopped by lead, concrete
Charge/Mass:
0/0
With Gamma,
Think ENERGY
•See Radiation Movie
*

Types of Radioactive Decay (cont.)
Specials

Positron Emission (also called Beta positive decay)
A positron is exactly like an electron in mass and charge
force except with a positive charge.
Charge/Mass: 1+ / 0 amu
It is formed when a proton breaks into a neutron with
mass and no charge
= positron (no mass and the positive charge)
Positron emission is most common in lighter elements
with a low neutron to proton ratio.
Specials (cont)

Electron Capture
A captured electron joins with a proton in the nucleus to
form (change to) a neutron.
= one less proton, turned into a neutron.
Charge/Mass: changes from +  0 / same
Electron capture is common in larger elements with a
low neutron to proton ratio.
Decay of Uranium
Uranium
*
Reminder:
Nuclear Equations

Equation that keeps track of the reaction’s components.
Alpha decay of Gold
185 Au  181 Ir + 4 a
79
77
2
Decay of Iodine
131 I  131 Xe +
53
54
0
-1b
Try a few (solve):
238 U  234 Th + ______
92
90
24 Na  ______ + 0 b
11
-1
Uranium:
Enriched Uranium: U-235
Low: 3-4% U-235 (remaining is U-238)
Reactor Grade
High: 90% U-235 (remaining is U-238)
Weapons Grade
Slightly (0.9%-2%) (replaces natural U in some reactors
Recovered: less U-235 than in natural occurring Uranium

Depleted Uranium: U-238

Remnants after enrichment
Less radioactive then natural Uranium
VERY Dense
-Useful for armor and penetrating weapons
NOTE: Depleted U-238 is still radioactive. Just LESS.
Uses for Radioactive materials:
Weapons:

Nuclear weapons
“Little Boy”
-Uranium, gun-type
-City of Hiroshima on
August 6, 1945
“Fat Man”
-Plutonium, Implosion
-City of Nagasaki on
August 9, 1945
Uses for Radioactive materials:
Smoke Detectors: Am-241
Gives off a particles
Ionizing energy (makes Ions)
Ionize smoke particles
Allows completion of a circuit (allows electricity to flow)
With a complete circuit, alarm sounds.

Cancer Treatment (BNCT)
Boron Neutron Capture Therapy
10
Patient is given Boron-10 ( B)
Using a neutron beam, doctors create thermal neutrons
which changes Boron-10 into excited Boron-11
Boron-11 decays, given off an a particle
a particle penetrated one-two cells deep,
Kills the cell(s)

Uses for Radioactive materials:
Carbon-14 dating
The radioactive C-14 method of dating is used to determine
the age of organic matter that is several hundred years to
approximately 50,000 yrs old.

C-14 is continually formed in nature by the interaction of
neutrons with N-14 in the Earth’s atmosphere.
The neutrons required for this reaction are produced by
cosmic rays interacting with the atmosphere.

C-14, along with non-radioactive C-13 and C-12, is
converted into CO2 and assimilated by plants and organisms.
When plant or animal dies, it no longer acquires carbon.
C-14 begins to decay.

Nucleosynthesis:
How were/are elements formed?
Nucleosynthesis (cont.):
How were/are heavier elements formed?
Fusion with increasingly larger and larger elements


+g
4
2He
+ 42He 
4
2He
+ 84Be  126C + g
8
4Be
Elements present in stars (depends on its size)

H, He, C, O, Ne, Mg + other heavier elements
Larger stars (greater gravitational energies), heavier
elements
Each “layer” acts to fuel the next “layer”.
H  He; He  C; C  O; O  Ne; Ne  Mg; Mg  Si;
Si  Fe

Heavier elements are created in supernovas
(exploding stars).